Pressure sensors are frequently embodied as semiconductor sensors. These are based, as a rule, on silicon. In such case, pressure sensor chips can be formed in a simple manner, and include, most often, a substrate and a thin bending plate formed therein by microstructuring and provided with integrated measuring elements. A pressure acting on the thin bending plate effects a pressure-dependent deflection. The integrated measuring elements react with a resistance change, which is registered as an electrical measurement signal. This provides a signal for additional processing and evaluation.
The sensor elements are embodied, in such case, as a rule, as piezoresistive elements, especially as resistances. It is known to manufacture these elements in silicon with the assistance of doping methods, such as, for example, diffusion or implantation. Thus, the piezoresistive elements can be embodied, for example, as a p conducting region in an n conducting silicon substrate.
Pressure sensors are exposed during operation to high loadings, such as, for example, pressure pulses, durably high pressures and strong temperature fluctuations. Pressure pulses and durably high pressures can lead to material fatigue and finally to fracture of the membrane and therewith to failure of the sensor.
Membrane fracture detection is, in such case, difficult, since an incorrect measured value can likewise be caused by other problems in the sensor, in the circuit or other used element or, in given cases, not be recognized at all.
For compensating the temperature dependence of the measurement signal, according to the state of the art, the temperature is determined by measuring a resistance change, for example, that of the measuring elements interconnected in a Wheatstone bridge. In such case, using the measured temperature and the earlier determined temperature dependence of the measurement signal, the temperature influence can be compensated. This technique has the disadvantage that also a pressure change leads to a resistance change. This makes a direct correlation between resistance change and temperature change difficult.
Moreover, a temperature coefficient of resistances used for measuring is selected in such a manner that, for a temperature range for which the sensor is designed, the temperature coefficient does not change sign. Upon a sign change, application of the bridge circuit for temperature determination becomes impossible, since the temperature must be determined injectively.
Besides compensating the temperature dependence of the measurement signal, known in the state of the art is also to protect against disturbing influences. For this, a shielding in the form of a conductive cover layer is applied on the membrane surface, in order to suppress external electrical disturbing influences, to the extent that the cover layer lies at the same potential as the substrate. For improving the stability of the output signal, consequently, frequently electrical shielding is applied, with which at least the sensor elements are covered. Metal shielding, however, likewise influences the measurement signal, so that, especially at high temperatures, non-reproducible deviations of the measurement signal can occur.
A material frequently applied for electrical shielding is doped polysilicon. Many material properties of polysilicon are very similar to those of single crystal silicon. These material properties include, among others, the thermal expansion coefficient, the hardness, as well as the modulus of elasticity and the shear modulus. Applied as doping material for polysilicon are, for example, boron or phosphorus.
The doping occurs, for example, by diffusion, implantation or by addition of a gas during deposition of the cover layer serving as shielding. Since the cover layer serves for temperature determination, a low doping is advantageous, in order to achieve a high resistance. A low doping has additionally a stronger temperature dependence than a high doping and produces, thus, a signal with sufficient range for temperature determination.
Advantageous for the sensor elements is, in contrast, a high doping, since this provides a lesser temperature dependence. Thus, the sensor elements cannot be used optimally simultaneously for pressure and temperature measurement.